Lunar swirls are recognized as bright, wavy patterns on the Moon's surface, bright enough to be seen through a backyard telescope. Many think they look like brush strokes in abstract painting. But these are not just artistic details: NASA images show that some strands of lunar swirls stretch for hundreds of kilometers.

Lunar swirls have long puzzled scientists, but recent modeling and spacecraft data shed new light on this mysterious phenomenon. Data shows that the rocks within the swirls are magnetized, causing deflection or redirection of solar wind particles that constantly bombard the Moon. Nearby rocks take the hits instead. Over time, neighboring rocks darken due to chemical reactions caused by collisions, while the swirls remain bright.

How did the rocks within the lunar swirls become magnetized? The Moon today has no magnetic field. No astronaut or rover has yet visited a lunar swirl to investigate this phenomenon.

Impacts and magnetic anomalies
“Impacts could trigger these kinds of magnetic anomalies,” said Michael J. Krawczynski, an associate professor of Earth, Environmental, and Planetary Sciences at Washington University in St. Louis. He notes that meteorites regularly deliver iron-bearing materials to the Moon's surface. “But there are some swirls where we’re not sure how an impact could create that shape and size.”

Krawczynski believes it is more likely that something else locally magnetized the swirls.

Theories about subsurface lava
“Another theory is that you have lavas beneath the surface that slowly cool in a magnetic field and create a magnetic anomaly,” said Krawczynski, who designed experiments to test this explanation. His results were published in the journal Journal of Geophysical Research: Planets.

Krawczynski and first author Yuanyuan Liang, who recently earned a doctorate in Earth, Environmental, and Planetary Sciences at Washington University, measured the effects of different combinations of atmospheric chemistry and magma cooling rates on a mineral called ilmenite to see if they could produce a magnetizing effect.

“Earth rocks are very easily magnetized because they often contain tiny particles of magnetite, which is a magnetic mineral,” said Krawczynski. “Many terrestrial studies that have focused on things with magnetite are not applicable to the Moon, where you do not have this hyper-magnetic mineral.”

But ilmenite, which is abundant on the Moon, can also react and form iron metal particles, which can be magnetized under the right conditions, Krawczynski and his team discovered.

Experimental results
“The smaller particles we worked with appeared to create stronger magnetic fields because the surface-to-volume ratio is greater for smaller particles compared to larger particles,” said Liang. “With more exposed surface, smaller particles more easily undergo the reduction reaction.”

“Our analog experiments showed that under lunar conditions we could create material that could be magnetized. So it is possible these swirls are caused by subsurface magma,” said Krawczynski, who is also a researcher at the university's McDonnell Center for Space Sciences.

Future missions
Determining the origin of lunar swirls is considered key to understanding the processes that have shaped the Moon's surface, the history of the Moon's magnetic field, and even how planetary and lunar surfaces in general affect the space environment surrounding them.

This study will help interpret data collected by future missions to the Moon, especially those exploring magnetic anomalies on the lunar surface. NASA plans to send a rover to the lunar swirl area known as Reiner Gamma in 2025 as part of the Lunar Vertex mission.

“If you are going to create magnetic anomalies by the methods we describe, then the subsurface magma must have a high titanium content,” said Krawczynski. “We have seen hints of this iron metal-forming reaction in lunar meteorites and Apollo samples. But all these samples are surface lava flows, and our study shows that cooling beneath the surface would significantly enhance these metal-forming reactions.”

For now, his experimental approach is the best way to test predictions about how unseen lava could be causing the magnetic effects of the mysterious lunar swirls.

“If we could only drill down, we could see if this reaction is occurring,” said Krawczynski. “That would be great, but it is not currently possible. For now, we are limited to the surface.”

Source: Washington University, St. Louis